The present invention relates to hot melt adhesives, which are widely used for various applications. Hot melt adhesives generally comprise materials that can be conveniently applied by extrusion or wheel application of the adhesive composition at elevated temperatures onto a workpiece to which it is desired to fix another workpiece.
Hot melt adhesives to date have had less than desirable thermal stability. That is, prior hot melt adhesives typically have a substantial viscosity change over time, darken significantly over a relatively short period of time, and produce char, skin or gel.
It is also noted that traditional hot melt adhesives have an unpleasant odor. Repeated use of such odiferous adhesives can result in a very unpleasant work environment.
Further, hot melt adhesives to date have poor bonding performance over a range of temperatures. That is, they lack satisfactory adhesiveness over a range of temperatures.
One application for hot melt adhesives is book binding Generally, there are two different methods for manufacturing book blocks. The “one-shot” method involves the application of an adhesive directly onto the spine of the book block. The “two-shot” method involves (1) the application of a primer composition onto the spine, and then (2) the application of an adhesive composition to the surface of the dried primer composition.
Water-based adhesives have typically been used as the primer composition in book binding. However, these adhesives have a slow rate of set that can be a detriment to the production rate of the binders. While dryers and heat have been employed to facilitate drying of water-based primers, exposure to heat typically causes the water-based primer to skin over and trap moisture within the formed polymeric membrane. The trapped water in the interior of the water based primer layer can vaporize, causing the membrane to lift and form large blisters that break and burn under heat. On the other hand, if the water-based primer is not sufficiently dried, it will cause splattering and blistering upon application of a second-shot hot melt adhesive at application temperatures ranging from about 100° C. to about 200° C. due to the remaining water present in the water based primer.
Thus, hot melt adhesives are more and more commonly used as primer adhesives in the two-shot method because they improve production speed, reduce the cost of binding due to the removal of the drying units, and eliminate the drying concerns associated with water based adhesive primers. One drawback, however, of hot melt adhesives available to date has been an inability to provide aggressive adhesion to a variety of paperstocks.
Therefore, there is a need in the art for an adhesive composition that has good thermal stability in that it has little or no viscosity change, little or moderate color change, and no charring, skinning, or gelling.
There is also a need for an adhesive composition that has less odor and a less unpleasant odor than traditional hot melt adhesives.
Further, there is a need for an adhesive composition that has superior bonding performance over a wide range of temperatures.
In addition, there is a need for an adhesive composition that provides aggressive adhesion to a variety of paperstocks in the area of book binding.